The mystery appendage that shocked a customer who bit into a McDonald's Sausage McMuffin is not a lizard, but a chicken blood vein, preliminary laboratory tests have found.

In a statement issued on Friday, senior communications manager Kevin Lim said the sample its staff collected from Facebook user Sandy Sand's home on Wednesday was found to be "100% chicken".

"The vein is white in colour because of the Halal slaughtering process where blood had to be drained," he explained. "It is commonly found in chicken meat products and is safe to consume."

Lim added that the McDonald's lab report has been shared with the appropriate authorities.

The fast-food outlet's chicken sausage patties, according to Lim, are made from minced dark and white chicken meat, and are imported from Malaysia.

"We have also traced back the product's production process and no product from the same batch had any quality issues reported," he added.

Earlier on Wednesday, the female customer discovered what she thought was the tail of a baby lizard sticking out of her Sausage McMuffin, after buying the burger from the Ang Mo Kio drive-through branch.

Sandy posted the photo on the fast food restaurant’s Facebook page at noon, leaving a Facebook post that said, “How gross is tat! (sic)? How r u going to answer for tis! (sic)??”

Netizens were quick to label the shock discovery as the "McLizard" burger.

A reader notified Yahoo Singapore this photo of an eaten Sausage McMuffin with the tail of a baby lizard sticking out.

The photo was snapped on Wednesday, by a McDonald customer known by her Facebook name as Sandy Sand, who discovered the baby lizard from the burger, which she bought in the morning, from the Ang Mo Kio drive-through branch.

Sandy posted the photo on the fast food restaurant’s Facebook page at noon, leaving an angry message that said, “How gross is tat!? How r u going to answer for tis!??”

The photo post already has about 100 shares and counting.

McDonalds Singapore replied about six hours later saying, “Dear Sandy, we sincerely apologise for your experience. We would like to assure you that food quality and safety is of utmost importance to us and we will be investigating this matter.”

Sandy was then asked to leave them a private message so that customer care staff can contact her directly.

Yahoo Singapore has contacted McDonalds Singapore for an official statement.